As the interest in natural gas as a source of hydrocarbons has risen, the desirability of converting methane, the major element of natural gas, into more easily handled products, such as olefins, aromatics and higher hydrocarbons, has greatly increased. Various processes exist for converting methane into such hydrocarbons, but processes for the direct conversion of methane into light olefins and higher hydrocarbons would be highly useful. Such processes, however, have required special catalysts which can affect such conversion.
For example, according to Burch et al, "Comparative Study of Catalysts for the Oxidative Coupling of Methane," Applied Catalysts, 43 (1988), pp. 105-116, various oxides are disclosed as catalysts for methane coupling, and, in fact, comparison is made in this article of various such oxide materials in terms of activity and selectivity, including magnesium oxide and various mixed oxides, including alkali metals which are said to alter the product distribution obtained therewith. According to Otsuka, "Direct Conversion of Methane to Higher Hydrocarbons," Sekiyu Gakkaishi, Vol. 30, No. 6, 1987, pp. 385-396, the rare earth metal oxides are disclosed for such direct conversion processes. Furthermore, the addition of alkali metals and in particular alkali metal halides such as LiCl is said to enhance the selectivity of these processes for the production of ethane. The lanthanide oxides are particularly described in this article. In Sofranko et al, "The Oxidative Conversion of Methane to Higher Hydrocarbons," Journal of Catalysis, Vol. 103 (1987), pp. 1-9, there is a further disclosure of transition metal oxides for use in these processes, including manganese, indium, germanium, antimony, tin, bismuth, and lead oxides, as coupling catalysts which give from 10 to 50% selectivity to higher hydrocarbons, including the use of a silica support therefor.
A large class of ternary oxides having the general formula A.sub.2 B.sub.2 O.sub.7, where A and B are metals, are known to adopt the structure of the natural mineral pyrochlore, (Na,Ca)(Nb,Ta).sub.2 O.sub.6 (OH,F). Subramanian et al, Prog. Solid St. Chem., 15, 55-143 (1983), have comprehensively reviewed the field of pyrochlore compounds, including the use of these materials in various electrical processes. The Subramanian et al article thus discloses that pyrochlore oxides are normally made by reaction of the constituent oxides at high temperatures, usually greater than about 800 C. Typically, the products produced thereby have low surface area and comprise crystals which are about 10 microns and up, and which have surface areas of less than about 1 m.sup.2 /g. Compositions with low surface area are usually not effective catalysts.
U.S. Pat. No. 4,124,539 discloses lead enriched pyrochlore compounds which contain ruthenium, iridium and mixtures thereof. The compositions are prepared by solid state reaction of a lead source and a ruthenium and/or an iridium source at temperatures below about 600.degree. C. in an oxygen environment. The '539 patent discloses that the compositions have a surface area of about 9 m.sup.2 /g, and the patent states that they have application in electrochemical processes, such as electrocatalysis.
U.S. Pat. No. 4,163,706 discloses bismuthrich pyrochlore-type compounds for use in the same types of processes. This material, however, was said to have a surface area of 178 m.sup.2 /g.
U.S. Pat. No. 4,129,525 also discloses lead-enriched and bismuth-enriched pyrochlore compounds which contain ruthenium, iridium and mixtures thereof. The pyrochlore oxides are prepared by reaction of lead and/or bismuth cations with ruthenium and/or iridium cations in aqueous solution. The pyrochlore oxide is precipitated from a liquid alkaline medium having a pH of about 13.5 in an oxygen environment at a temperature below about 200.degree. C. The reaction conditions provide a product with a high surface area, i.e., around 60-250 m.sup.2 /g. In a similar vein are U.S. Pat. Nos. 4,146,458, relating to such pyrochlore compounds containing Ru, Rh, Ir, Os, Pt, Ru-Pb, and Ir-Pb mixtures; and 4,192,780, relating to methods of preparing pyrochlore compounds by the precipitation of the metal cations from an aqueous solution in a liquid alkaline medium in the presence of an oxygen source at below 200.degree. C., resulting in compounds of 25-150 m.sup.2 /g surface area, which is said to be generally useful in a catalytic or electrocatalytic environment. Similar disclosures are included in U.S. Pat. Nos. 4,203,871; 4,225,469; 4,434,031; and 4,440,670.
While moderate selectivities and conversions have been obtained with these oxides, there has been a significant need for improved catalysts for direct methane conversion.